<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>11_固定定位全屏遮罩</title>
    <style>
        .mask{
            background-color: rgba(0,0,0,.4);
            position: fixed;
            top: 0;
            bottom: 0;
            left: 0;
            right: 0;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="mask"></div>
    <div>

    </div>
</div>

</body>
</html>